The mouse uterus has provided a system for the study of estrogen since it contains estrogen receptors and depends on estrogen stimulation for action of physiological functions. We have previously purified an estrogen-induced secretory protein from mouse uterine luminal fluid by CM-Affi-Gel blue and reversed phase high-performance liquid chromatography. This protein shows a Mr about 70,000 both by SDS-polyacrylamide (with or without Beta-mercaptoethanol) gel electrophoresis and by gel filtration column chromatography indicating that it exists as a single chain polypeptide. Further analysis of the protein revealed that it is highly basic (pI greater than 10) and is a glycoprotein. The in vitro incorporation of 35S-methionine into uterine proteins revealed that estrogen treatment of immature mice stimulates both synthesis and secretion of the 70 kDa protein. Rabbit polyclonal antibody raised against the purified 70 kDa protein demonstrated specificity for the 70 kDa protein by "Western Blot" analysis. An enzyme-linked immunosorbent assay with polyclonal antibody has been used to determine the tissue distribution of the protein. Tissues such as lung, brain, spleen, muscle, intestine, liver, kidney, and ovary of estrogen-treated mice did not have detectable amounts of the 70 kDa protein. Immunoreactivity was present in uterine and vaginal tissues from estrogen-treated animals. The 70 kDa protein was not induced by testosterone or progesterone. Currently, a cDNA to the message for this protein has been isolated, sequenced, and found to be a member of the transferrin gene family. A comparison of the properties of the uterine protein and serum transferrin indicates that they are distinct proteins. This has been confirmed by isolation of a cDNA to authentic mouse serum transferrin. Although the function of this uterine secretory protein is unknown, the possible involvement of this protein in the iron transport and the growth promotion is currently under investigation.